The St Andrew’s Cross spider, Argiope keyserlingi, is very common in our gardens and it is named for its trademark, an “x” cross built into its web, this being the symbol of the Scottish patron saint. (Why? Find out here.)
This mid-sized female, however, wasn’t going to stop with an “x” but had added half of a central vertical stroke and a hint of the other half when I saw her yesterday. Her web is the same today, so that must be how she likes it. Why? No-one knows. In fact, no-one knows why spiders add any of these decorations to their webs.
The decoration, whether it’s a cross (as this ought to be), a loose squiggle (like this, created by a juvenile of the same species, or this, created by an adult of a related species) or any other shape, is called a stabilimentum. The new Whyte & Anderson Field Guide says, “proposed functions have included providing camouflage, making the web more conspicuous to prevent destruction, or attracting insects by reflecting UV light.”
Other orb-weaving spiders add different kinds of solid-looking components to their webs. Cyclosa species, for instance, construct a bar of prey debris across the centre of their web, in which they very deliberately camouflage themselves, but there is no guarantee that this behaviour is related to the Argiope species’ construction of a stabilimentum.